Wavelength division multiplexed optical communication system having variable channel spacings

ABSTRACT

Consistent with the present disclosure, data, in digital form, is received by a transmit nodes of an optical communication, and converted to analog signal by a digital-to-analog converter (DAC) to drive a modulator. The modulator, in turn, modulates light at one of a plurality of wavelengths in accordance with the received data. The modulated light is then transmitted over an optical communication path to a receive node. At the receive node, the modulated optical signal, as well as other modulated optical signals are supplied to a photodetector circuit, which receives additional light at one of the optical signal wavelengths from a local oscillator laser. An analog-to-digital converter (ADC) is provided in the receive node to convert the electrical signals output from the photodetector into digital form. The output from the ADC is then filtered in the electrical domain, such that optical demultiplexing of individual channels is unnecessary.

BACKGROUND

Wavelength division multiplexed (WDM) optical communication systems areknown in which multiple optical signals or channels, each having adifferent wavelength, are combined onto an optical fiber. Such systemstypically include a laser associated with each wavelength, a modulatorconfigured to modulate the optical signal output from the laser, and anoptical combiner to combine each of the modulated optical signals. Suchcomponents are typically provided at a transmit end of the WDM opticalcommunication system to transmit the optical signals onto the opticalfiber. At a receive end of the WDM optical communication system, theoptical signals are often separated and converted to correspondingelectrical signals that are then processed further.

Preferably, the information carrying capacity of an opticalcommunication system should be optimized to carry a maximum amount ofdata over a maximum length of optical fiber. In optimizing the capacity,however, certain trade-offs are often made. For example, certainmodulation formats may be employed to modulate the optical signals tocarry data at higher rates. Such higher rate modulation formats,however, are typically more susceptible to noise, and, therefore, maynot be used in transmission of optical signals over relatively longdistances.

Capacity may be further increased by transmitting a relatively largenumber of channels over the optical fiber. Trade-offs, however, areencountered here as well. For example, when increased numbers ofchannels are provided, each channel is typically provided spectrallyclose to each other, thereby increasing error rates due to cross-talk,as well as non-linear effects, such as cross-phase modulation (XPM).Moreover, the susceptibility of channel to cross-talk, non-lineareffects, and noise are often wavelength dependent. Thus, a channel atone wavelength may have more or fewer errors due to XPM or othernon-linear effects compared to a channel at another wavelengths.Accordingly, a maximum capacity may be achieved by optimizing the abovenoted parameters, such as modulation format, distance, and channelspacing. Such optimized capacity may require non-uniformly spacedchannels, for example.

Optical demultiplexers are often employed to separate or demultiplex thecombined optical signals. Typically, such optical demultiplexers includeoptical components that have a fixed bandwidth to select optical signalshaving a particular wavelength. Accordingly, since different WDM opticalcommunication systems extend over different lengths of fiber, includedifferent types of fiber, and may have other differing characteristics,optical demultiplexers must be tailored for each WDM opticalcommunication system if each such system is to have optimized capacity.As a result, such tailored optical demultiplexers are typicallyexpensive.

Moreover, the wavelengths associated with each optical signal are oftenuniformly spaced from each other so as to conform to a so-calledstandardized “grid.” In one such wavelength grid, standardized by theInternational Telecommunications Union (ITU), wavelengths are spectrallyspaced from one another by 50 GHz. Such 50 GHz spaced wavelengths orgrid wavelengths include 1569.18 nm, 1568.36 nm, 1567.54 nm, etc.Typically, systems that transmit optical signals having wavelengthsconforming to the ITU grid do not transmit optical signals havingwavelengths between the grid wavelengths. Thus, such systems may nothave a channel spacing or other optimized parameters to provide maximumcapacity.

An optical communication system is therefore needed that has flexiblechannel spacing and bandwidth so that the capacity of such a system canbe optimized for a given fiber type and distance, as well as othersystem parameters.

SUMMARY

Consistent with an aspect of the present disclosure, an apparatus isprovided that comprises a plurality of optical transmitters, each ofwhich being configured to supply a corresponding one of a plurality offirst optical signals. Each of the plurality of first optical signalshas a corresponding one of a plurality of wavelengths and carries acorresponding one of a plurality of data streams. The apparatus alsoincludes an optical combiner configured to combine the plurality offirst optical signals onto an optical path, and a photodiode configuredto receive a portion of each of the plurality of first optical signalsand supply a first electrical signal. A local oscillator laser is alsoprovided, such that at least a portion of a second optical signal outputfrom the local oscillator is also supplied to the photodiode. Inaddition, circuitry is provided that is circuitry configured to receivethe first electrical signal and supply a second electrical signal inresponse to the first electrical signal. The circuitry includes anelectronic filter having a variable bandwidth, and the second electricalsignal carries one of the plurality of data streams.

Consistent with an additional aspect of the present disclosure, anapparatus is provided that comprises a digital signal processor circuitconfigured to receive input data. The digital signal processor isconfigured to sample or receive the input data at a first sampling orbaud rate, and spectrally shape the input data to supply spectrallyshaped data as a first plurality of data samples at the first samplingrate or baud rate. An interpolation circuit is also provided thatreceives the first plurality of data samples at the first sampling rateand outputs the spectrally shaped data as a second plurality of datasamples at a second sampling rate greater than the first sampling rate.A digital-to-analog converter circuit is also provided that isconfigured to receive the second plurality of data samples and generatean analog signal. The apparatus further includes a modulator circuit anda laser configured to supply light to the modulator circuit, themodulator circuit being configured to modulate the light to supply amodulated optical signal in response to the analog signal.

Moreover, consistent with a further aspect of the present disclosure, anapparatus is provided that comprises a photodiode that receives aportion of each of a plurality of optical signals, each of which beingmodulated in accordance with a corresponding one of a plurality of datastreams, and each having a corresponding one of a plurality ofwavelengths. The photodiode supplies an electrical output, such thateach of the plurality of optical signals is supplied by a correspondingone of a plurality of transmitters. In addition, a low-pass filter isprovided that supplies a filtered output in response to the electricaloutput, and an analog-to-digital converter is provided that isconfigured to sample the filtered output at a first sampling rate togenerate a plurality of first data samples. In addition, aninterpolation circuit is provided that is configured to receive theplurality of first data samples and supply a plurality of second datasamples at a second sampling rate less the first sampling rate. Further,a digital signal processor circuit is provided that is configured toreceive the plurality of second data samples.

Consistent with an additional aspect of the present disclosure, a systemis provided that comprises a first transmitter including a first digitalsignal processor circuit configured to receive input data. The digitalsignal processor is configured to sample the input data at a firstsampling rate, and spectrally shape the input data to supply spectrallyshaped data as a first plurality of data samples at the first samplingrate. The transmitter also includes a first interpolation circuit thatreceives the first plurality of data samples at the first sampling rateand outputs the spectrally shaped data as a second plurality of datasamples at a second sampling rate greater than the first sampling rate.In addition, the transmitter includes a digital-to-analog convertercircuit configured to receive the second plurality of data samples andgenerate an analog signal. Further, a modulator circuit, and a laser isprovided that is configured to supply light to the modulator circuit.The modulator circuit is configured to modulate the light to supply afirst modulated optical signal in response to the analog signal. Theapparatus also includes a second transmitter that supplies a secondmodulated optical signal. The apparatus also includes a combinerconfigured to combine the first modulated optical signal with the secondmodulated optical signal onto an optical communication path. Further,the system includes a receiver coupled to the optical communicationpath. The receiver includes a photodiode that receives portions thefirst and second modulated optical signals. The photodiode supplies anelectrical output to a low-pass filter, which, in turn, supplies afiltered output in response to the electrical output. The receiver alsoincludes an analog-to-digital converter configured to sample thefiltered output at the second sampling rate to generate a plurality ofthird data samples, and a second interpolation circuit configured toreceive the plurality of third data samples and supply a plurality offourth data samples at a third sampling rate less than the secondsampling rate. The receiver also includes a second digital signalprocessor circuit configured to receive the plurality of fourth datasamples.

Consistent with a further aspect of the present disclosure, an apparatusis provided that comprises a digital signal processor circuit configuredto receive input data. The digital signal processor is configured tosample the input data at a first sampling rate, and spectrally shape theinput data to supply spectrally shaped data as a first plurality of datasamples at the first sampling rate. An interpolation circuit is alsoprovided that receives the first plurality of data samples at the firstsampling rate and outputs the spectrally shaped data as a secondplurality of data samples at a second sampling rate greater than thefirst sampling rate. In addition, a digital-to-analog converter circuitis provided that is configured to receive the second plurality of datasamples and generate an analog signal. Further, a low-pass filter isprovided, such that the analog signal is supplied to the low passfilter, and the low-pass filter outputs a filtered signal in response tothe analog signal. The low pass filter has an associated roll-offfactor, the roll-off factor being adjusted in response to a controlinput. In addition, a modulator circuit and a laser are provided, suchthat the laser supplies light to the modulator circuit, and themodulator circuit is configured to modulate the light to supply amodulated optical signal in response to the filtered signal.

Consistent with a further aspect of the present disclosure, an apparatusis provided that comprises a forward error correction (FEC) encodercircuit configured to receive input data, such that, in response to afirst control input, the FEC encoder circuit generates first errorcorrecting bits, and, in response to a second control input, the FECencoder generates second error correcting bits, a number of the seconderror correcting bits being greater than a number of the first errorcorrecting bits. The apparatus further includes a plurality of opticaltransmitters, each of which being configured to supply a correspondingone of a plurality of first optical signals. Each of the plurality offirst optical signals has a corresponding one of a plurality ofwavelengths and carries a corresponding one of a plurality of datastreams, one of the plurality of data streams including one of the firsterror correcting bits and the second error correcting bits. An opticalcombiner is also provided that is configured to combine the plurality offirst optical signals onto an optical path. In addition, a photodiode isprovided that is configured to receive a portion of each of theplurality of first optical signals and supply a first electrical signal.Further, a local oscillator laser is provided, such that at least aportion of a second optical signal output from the local oscillator issupplied to the photodiode. Moreover, circuitry is provided that isconfigured to receive the first electrical signal and supply a secondelectrical signal in response to the first electrical signal. Thecircuitry includes an electronic filter having a first bandwidth whenthe first error correcting bits are included in said one of theplurality of data streams and a second bandwidth when the second errorcorrecting bits are included in said one of the plurality of datastreams. The second bandwidth is spectrally wider than the firstbandwidth, and the second electrical signal carries one of the pluralityof data streams.

Consistent with an additional aspect of the present disclosure, anapparatus is provided that includes a substrate and an optical splitterprovided on the substrate. The optical splitter has an input and aplurality of outputs, such that the optical splitter receives awavelength division multiplexed (WDM) optical signal including pluralityof optical signals, each having a corresponding one of a plurality ofwavelengths. Each of the plurality of outputs of the optical splittersupplies a corresponding one of a plurality of first portions of the WDMoptical signal. A plurality of local oscillator lasers are also providedon the substrate, as well as a plurality of optical hybrid circuits.Each of the plurality of optical hybrid circuits has a first inputcoupled to a corresponding one of the plurality of outputs of theoptical splitter and a second input configured to receive a firstportion of light supplied by a corresponding one of the plurality oflocal oscillator lasers. Each of the plurality of optical hybridcircuits has an output that supplies a corresponding one of a pluralityof second portions of the WDM optical signal and a second portion of thelight supplied by a corresponding one of the plurality of localoscillator lasers, each of the plurality of second portions of the WDMoptical signal including light at each of the plurality of wavelengths.Further, a plurality of photodiodes is provided on the substrate. Eachof the plurality of photodiodes receives a respective one of theplurality of second portions of the WDM optical signal and the secondportion of the light supplied by a corresponding one of the plurality oflocal oscillator circuits. Each of the plurality of photodiodes suppliesa corresponding one of a plurality of electrical signals.

Consistent with an additional aspect of the present disclosure, anapparatus is provided that includes a wavelength selective switch havingan input that receives a plurality of optical signals, each of whichhaving a corresponding one of a plurality of wavelengths. The wavelengthselective switch has a plurality of outputs, each of which supplying acorresponding one of a plurality of groups of the plurality of opticalsignals. The apparatus also includes a plurality of photonic integratedcircuits, each of which being configured to receive a corresponding oneof the plurality of band of the plurality of optical signals. One ofplurality of photonic integrated circuits includes a substrate and anoptical splitter provided on the substrate. The optical splitter has aninput and a plurality of outputs, such that the optical splitterreceives one of the plurality of groups, which includes a subset of theplurality of optical signals. Each optical signal within the subset ofthe plurality of optical signals having respective one of a subset ofthe plurality of wavelengths, each of the plurality of outputs of theoptical splitter supplying a corresponding one of a plurality of firstportions of said one of the plurality of groups. A plurality of localoscillator lasers are also provided on the substrate, as well as aplurality of optical hybrid circuits. Each of the plurality of opticalhybrid circuits has a first input coupled to a corresponding one of theplurality of outputs of the optical splitter and a second inputconfigured to receive a first portion of light supplied by acorresponding one of the plurality of local oscillator lasers. Each ofthe plurality of optical hybrid circuits has an output that supplies acorresponding one of a plurality of second portions of said one of theplurality of groups and a second portion of the light supplied by acorresponding one of the plurality of local oscillator lasers. Each ofthe plurality of second portions of said one of the plurality of groupsincludes light at each of the subset of the plurality of wavelengths. Inaddition, a plurality of photodiodes is provided on the substrate, eachof which receiving a respective one of the plurality of second portionsof said one of the plurality of band and the second portion of the lightsupplied by a corresponding one of the plurality of local oscillatorcircuits. Each of the plurality of photodiodes supplies a correspondingone of a plurality of electrical signals.

Consistent with an additional aspect of the present disclosure, anoptical de-interleaver is provided that has an input for receiving aplurality of optical signals, each of which having a corresponding oneof a plurality of wavelengths. The optical de-interleaver has aplurality of outputs, each of which supplying a corresponding one of aplurality of groups of the plurality of optical signals. The apparatusalso includes a plurality of photonic integrated circuits, each of whichbeing configured to receive a corresponding one of the plurality of bandof the plurality of optical signals. One of plurality of photonicintegrated circuits includes a substrate and an optical splitterprovided on the substrate. The optical splitter has an input and aplurality of outputs, such that the optical splitter receives one of theplurality of groups, which includes a subset of the plurality of opticalsignals. Each optical signal within the subset of the plurality ofoptical signals having respective one of a subset of the plurality ofwavelengths, each of the plurality of outputs of the optical splittersupplying a corresponding one of a plurality of first portions of saidone of the plurality of groups. A plurality of local oscillator lasersare also provided on the substrate, as well as a plurality of opticalhybrid circuits. Each of the plurality of optical hybrid circuits has afirst input coupled to a corresponding one of the plurality of outputsof the optical splitter and a second input configured to receive a firstportion of light supplied by a corresponding one of the plurality oflocal oscillator lasers. Each of the plurality of optical hybridcircuits has an output that supplies a corresponding one of a pluralityof second portions of said one of the plurality of groups and a secondportion of the light supplied by a corresponding one of the plurality oflocal oscillator lasers. Each of the plurality of second portions ofsaid one of the plurality of groups includes light at each of the subsetof the plurality of wavelengths. In addition, a plurality of photodiodesis provided on the substrate, each of which receiving a respective oneof the plurality of second portions of said one of the plurality of bandand the second portion of the light supplied by a corresponding one ofthe plurality of local oscillator circuits. Each of the plurality ofphotodiodes supplies a corresponding one of a plurality of electricalsignals.

Consistent with an additional aspect of the present disclosure, anapparatus is provided that includes a first optical transmitterconfigured to supply a first optical signal that is modulated inaccordance with a first modulation format, the first optical signalhaving a first wavelength and carrying a first information stream. Asecond optical transmitter is provided which is configured to supply asecond optical signal modulated in accordance with a second modulationformat and carrying a second information stream. An optical combiner isalso provided that is configured to combine the first and second opticalsignals onto an optical path. In addition, a first photodiode isprovided that is configured to receive a first portion of each of thefirst and second optical signals and supply a first electrical signal.Further, a second photodiode is provided that is configured to receive asecond portion of each of the first and second optical signals andsupply a second electrical signal. Moreover, a local oscillator laser isprovided, such that portions of a third optical signal output from thelocal oscillator are supplied to the first and second photodiodes. Firstcircuitry is included to receive the first electrical signal and supplya third electrical signal in response to the first electrical signal.The first circuitry includes a first electronic filter having a variablebandwidth. The third electrical signal carries data associated with thefirst information stream. In addition, second circuitry is provided thatis configured to receive the second electrical signal and supply afourth electrical signal in response to the first electrical signal. Thesecond circuitry includes a second electronic filter having a variablebandwidth, and the fourth electrical signal carries data associated withthe second information stream.

Consistent with an additional aspect of the present disclosure, anapparatus is provided that comprises a plurality of opticaltransmitters. A first one of the plurality of optical transmitter isconfigured to supply a first optical signal modulated in accordance witha first modulation format in response to a first control signal suppliedto the first one of the plurality of optical transmitters. The first oneof the plurality of optical transmitters is also configured to supply asecond optical signal modulated in accordance with a second modulationformat in response to a second control signal supplied to the first oneof the plurality of optical transmitters. Also, a second one of theplurality of optical transmitters is configured to supply a thirdoptical signal modulated in accordance with the first modulation formatin response to a third control signal supplied to the second one of theplurality of optical transmitters. The second one of the plurality ofoptical transmitters also being configured to supply a fourth opticalsignal modulated in accordance with the second modulation format inresponse to a fourth control signal supplied to the second one of theplurality of optical transmitters. Also, an optical combiner is providedthat is configured to combine one of the first and second opticalsignals and one of the third and fourth optical signals onto an opticalpath. Each of the plurality of optical transmitters supplies acorresponding one of a plurality of optical signals, each of which has acorresponding one of a plurality of wavelengths. One of the first andsecond optical signals is a first one of the plurality of opticalsignals, and one of the third and fourth optical signals is a second oneof the plurality of optical signals. First and second adjacent ones ofthe plurality of wavelengths are separated from each other by a firstspacing, and second and third adjacent ones of the plurality ofwavelengths are separated from each other by a second spacing differentthan the first spacing.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the invention, as claimed.

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate one (several) embodiment(s) ofthe invention and together with the description, serve to explain theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an optical communication system consistent with anaspect of the present disclosure;

FIG. 2 illustrates a diagram of a transmit block consistent with anadditional aspect of the present disclosure;

FIG. 3 illustrates a portion of the transmit block shown in FIG. 2 ingreater detail;

FIG. 4 illustrates an example of an FIFO and interpolation filtercircuit consistent with the present disclosure;

FIG. 5 illustrates a portion of transmit photonic integrated circuitconsistent with the present disclosure;

FIG. 6 illustrates a receive block consistent with an aspect of thepresent disclosure;

FIG. 7 illustrates a portion of a receiver photonic integrated circuitconsistent with the present disclosure;

FIG. 8 illustrates a portion of the receive block shown in FIG. 6;

FIGS. 9 a and 9 b illustrate examples of bandwidth and filter spectra;

FIG. 10 illustrates an example of a optical communication systemconsistent with an additional aspect of the present disclosure;

FIG. 11 a illustrates an example of a bandwidth consistent with thepresent disclosure;

FIG. 11 b illustrates an example of a frame having an overheadconsistent with the present disclosure;

FIG. 11 c illustrates an example of a bandwidth consistent with thepresent disclosure;

FIG. 11 d illustrates an example of a frame consistent with the presentdisclosure;

FIG. 12 illustrates a further example of an optical communication systemconsistent with the present disclosure;

FIG. 13 illustrates an additional example of an optical communicationsystem consistent with the present disclosure;

FIG. 14 illustrates another example of an optical communication systemconsistent with the present disclosure;

FIGS. 15 a-15 c illustrate examples of filter and bandwidth spectraconsistent with the present disclosure;

FIG. 16 illustrates an example of a channel plan consistent with thepresent disclosure; and

FIG. 17 illustrates a flow chart in connection with a method consistentwith a further aspect of the present disclosure.

DESCRIPTION OF THE EMBODIMENTS

Consistent with the present disclosure, data, in digital form, isreceived by a transmit nodes of an optical communication, and convertedto analog signal by a digital-to-analog converter (DAC) to drive amodulator. The modulator, in turn, modulates light at one of a pluralityof wavelengths in accordance with the received data. The modulated lightis then transmitted over an optical communication path to a receivenode. At the receive node, the modulated optical signal, as well asother modulated optical signals may be intradyned in a known manner bycombination with light from a local oscillator at one of the opticalsignal wavelengths to provide a baseband optical signal. The basebandoptical signal, is supplied to a photodetector, which, in turn, suppliesan analog electrical signal (representative of a known “down-converted”signal), that may be amplified or otherwise processed, and provided toan an analog-to-digital converter (ADC). The ADC converts processedanalog electrical signals into digital form. The output from the ADC isthen further processed to output a copy of the data supplied to thetransmit node.

In particular, such processing may include filtering electrical signalsgenerated in response to the ADC outputs in order select data associatedwith one of the plurality of modulated optical signals. Since filteringis carried out electronically, i.e., in the electrical domain, insteadof optically, fixed optical demultiplexers are not required. Moreover,the electrical bandwidth of the data associated with or carried by theoptical signals may be readily tuned by circuitry in the transmit andreceive nodes, such that the optical signals carrying such data may bespaced closer to one another or may be adjusted to accommodate differentchannel bandwidths.

Typically, filtering is also carried out in the transmit node in orderto limit the electrical bandwidth of the data to be carried by theoptical signal to be substantially equal to an associated Nyquist limit.That is, the electrical bandwidth is filtered so that the Nyquistfrequency (half the sampling or symbol rate), is greater than suchelectrical bandwidth (or the maximum frequency component of the data).Such filtering may also be provided to minimize or eliminateinterference with an adjacent channel and may be carried out by a filterin a digital signal processor in the transmit node operating on samplesof the data at a first sampling rate. Such filtered samples may besupplied to the DAC, which, as noted above, supplies a correspondinganalog drive signal. The filtered samples have an associated bandwidth,which corresponds to the bandwidth of the channel and also constitutesthe bandwidth associated with the electronic filter of the digitalsignal processor. In addition to the channel bandwidth, however, the DACgenerates higher frequency harmonics through known “aliasing”. Suchhigher frequency harmonics are typically filtered with a low pass or“roofing” analog filter that filters the analog output of the DAC,otherwise such harmonics may create distortions that are sensed at thereceive node and cause errors in data output from the receive node.

If the DAC operates at the same sampling rate or data rate as the baudor symbol rate of the digital signal processor or the data rate of thedigital signal processor, higher frequency harmonics are generatedrelatively close in frequency to the channel bandwidth and cannot beeffectively filtered by the roofing filter. Accordingly, consistent withthe present disclosure, an interpolating circuit is provided tointerpolate the data output from digital signal processor so that theDAC can operate at a higher sampling or symbol rate than the DSP or ahigher data rate than the DSP. As a result, the higher frequencyharmonics can be more readily filtered by the roofing filter. Thus, theinterpolating circuit permits adequate filtering of the higher frequencyharmonics, which, in turn, facilitates electronic filtering at thereceive node with fewer errors.

Various examples of systems including electronic filtering consistentwith the present disclosure are discussed below.

Reference will now be made in detail to the present exemplaryembodiments of the present disclosure, which are illustrated in theaccompanying drawings. Wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or like parts.

FIG. 1 illustrates an optical link or optical communication system 100consistent with an aspect of the present disclosure. Opticalcommunication system 100 includes a plurality of transmitter blocks (TxBlock) 12-1 to 12-n provided in a transmit node 11. Each of transmitterblocks 12-1 to 12-n receives a corresponding one of a plurality of dataor information streams Data-1 to Data-n, and, in response to arespective one of these data streams, each of transmitter blocks 12-1 to12-n may output a group of optical signals or channels to a combiner ormultiplexer 14. Each optical signal carries an information stream ordata corresponding to each of data streams Data-1 to Data-n. Multiplexer14, which may include one or more optical filters, for example, combineseach of group of optical signals onto optical communication path 16.Optical communication path 16 may include one or more segments ofoptical fiber and optical amplifiers, for example, to optically amplifyor boost the power of the transmitted optical signals.

As further shown in FIG. 1, a receive node 18 is provided that includesan optical combiner or demultiplexer 20, which may include one or moreoptical filters, for example, optical demultiplexer 20 supplies eachgroup of received optical signals to a corresponding one of receiverblocks (Rx Blocks) 22-1 to 22-n. Each of receiver blocks 22-1 to 22-n,in turn, supplies a corresponding copy of data or information streamsData-1 to Data-n in response to the optical signals. It is understoodthat each of transmitter blocks 12-1 to 12-n has the same or similarstructure and each of receiver blocks 22-1 to 22-n has the same orsimilar structure.

FIG. 2 illustrates one of transmitter blocks 12-1 in greater detail.Transmitter block 12-1 may include a digital signal processor (DSP)including circuitry or circuit blocks CB1-1 to CB1-10, each of whichreceiving, for example, a corresponding portion of Data-1 and supplyinga corresponding one of outputs or electrical signals to 202-1 to 202-10to a circuit, such as application specific integrated circuit (ASIC)204. ASIC 204 include circuit blocks CB2-1 to CB2-10, which supplycorresponding outputs or electrical signals 204-1 to 204-10 to opticalsources or transmitters OS-1 to OS-2 provided on transmit photonicintegrated circuit (PIC) 205. As further shown in FIG. 2, each ofoptical sources OS-1 to OS-2 supplies a corresponding one of modulatedoptical signals having wavelengths λ1 to λ10, respectively. The opticalsignals are combined by an optical combiner or multiplexer, such asarrayed waveguide grating (AWG) 208, for example, and combined into aband or group of optical signals supplied by output 206-1.Alternatively, a known optical power multiplexer may be provided tocombine the optical signals. Optical sources OS-1 to OS-10 andmultiplexer 208 may be provided on substrate 205, for example. Substrate205 may include indium phosphide or other semiconductor materials.Although FIG. 2 illustrates ten circuit blocks CB1-1 to CB1-10, tencircuit blocks CB2-1 to CB2-10, and ten optical sources OS1-1 to OS-10,it is understood that any appropriate number of such circuit blocks andoptical sources may be provided. Moreover, it is understood, thatoptical sources OS-1 to OS-10, as well as multiplexer 208, may beprovided as discrete components, as opposed to being integrated ontosubstrate 205 as PIC 206. Alternatively, selected components may beprovided on a first substrate while others may be provided on one ormore additional substrates in a hybrid scheme in which the componentsare neither integrated onto one substrate nor provided as discretedevices.

FIG. 3 illustrates circuit block CB1-1 of DSP 202 and circuit blockCB2-1 of ASIC 204 in greater detail. First portions of Data-1 may beprocessed within DSP circuitry 202 (also referred to as “DSP” herein)and supplied to circuitry, such as digital filter 302 while a secondportion of Data-1 may be processed within DSP 202 and supplied todigital filter 304. Digital filters 302 and 304 may operate in a knownmanner such that modulated optical signals supplied by optical sourceOS-1, for example, have a desired spectral shape or bandwidth. Suchspectral shaping may be desirable in order to reduce interference withan adjacent channel, as noted above. In one example, DSP 202 and filters304 and 304 operate on samples (“first samples”) of the first and seconddata portions at a given sample or sampling rate or data rate. Digitalfilters 302 and 304 may include known raised-cosine filters implementedwith a Fast Fourier Transform (FFT). In addition, as generallyunderstood, digital filters 302 and 304 may have an associated“roll-off” factor (α). Consistent with the present disclosure, however,such “roll-off” may be adjustable or changed in response to differentcontrol inputs to filters 302 and 304. Such variable roll-off results indigital filters 302 and 304 having a variable or tunable bandwidth. Itis understood that the control inputs may be any appropriate signal,information, or data that is supplied to filters 302 and 304, such thatthe “roll-off” is changed in response to such signal, information ordata.

The filtered outputs (electrical signals) or filtered data supplied fromfilters 302 and 304 are supplied to FIFO and interpolation filter(circuit) blocks 606 and 308. As discussed in greater detail below,these circuit blocks interpolate the first samples and output secondsamples of the filtered data at a second sampling or data rate, which istypically higher than the first sampling or data rate. FIFO andinterpolation filter 306 outputs data samples 202-1 a and data samples202-1 b to DACs 310 and 312, respectively, and FIFO and interpolationfilter 308 outputs samples 202-1 c and 202-1 c to DACs 314 and 316,respectively.

A portion of FIFO and interpolation circuit or “interpolation circuit”306 supplying samples 202-1 a will next be described with reference toFIG. 4.

Interpolation circuit 306 includes a filter, such as a poly-phase filter(although another known filter, such as a finite impulse response (FIR)filter may be provided instead) and a memory, such as afirst-in-first-out memory (FIFO) 404. Although the memory is shown as aFIFO in the example FIG. 4, the memory may include other suitablememories. Interpolation circuit 306 may also include a voltagecontrolled oscillator (VCO) 408, as well as clock divider circuit 406.VCO 408 supplies a clock signal clk1 to DAC 310 and clock dividercircuit 406, and is used to control an output of FIFO 404. Clock dividercircuit 406, in turn, outputs a clock signal clk2 which is slower or hasa lower clock rate than clock signal clk1.

In operation, slower clock signal clk2 is supplied to filter 302 suchthat filter 302 outputs, in this example, 64 samples (302-a) per eachcycle of clock signal clk1. Poly-phase filter 402 receives the samplesoutput from output from filter 302 and outputs 96 interpolated samplesper cycle of clock signal clk1 to FIFO 404. An input of FIFO 404receives these 96 samples at a rate associated with clock signal clk2.At the output of FIFO 404, however, these samples (202-1 a) are outputto DAC 310 at a higher clock rate associated with clock signal clk1.Here, clock signal clk1 has a clock rate that is 3/2 times the clockrate of clock signal clk2. Thus, although 1.5 times the number ofsamples are input to FIFO 404, such samples are output from FIFO 404 at1.5 times the rate that they are input to FIFO 404. As a result, the netflow of data samples into and out of FIFO 404 may be the same.

Accordingly, as noted above, DAC 310 may operate at a higher sampling ordata rate than filter 302. That is, data is “up sampled” consistent withthis aspect of the present disclosure. An advantage associated with suchup-sampling will be discussed below with reference to FIG. 10.

It is noted, however, that the present disclosure is not limited to thenumbers of samples, sampling rates, clocks, and clock rates discussedabove. In addition, other circuitry in FIFO and interpolation filter 306similar to that shown in FIG. 4 may supply higher rate samples to DAC312. Further, it is understood that FIFO and interpolation filter 308has the same or similar structure of FIFO and interpolation filter 306.

Returning to FIG. 3, DACs 310 and 312 output corresponding analogsignals in response to output data samples 202-1 a and 202-1 b fromcircuit 306, and DACs 314 and 316 output corresponding analog signals inresponse to samples received from circuit 308. The analog signals outputfrom DACs 310 and 312 are filtered by low-pass or roofing filters 318and 320 to thereby remove, block or substantially attenuate higherfrequency components in these analog signals. Such high frequencycomponents or harmonics are associated with sampling performed by DACs310 and 312 and are attributable to known “aliasing.” The analog signaloutput from DACs 314 and 316 are similarly filtered by roofing filters322 and 324. The filtered analog signals output from roofing filters318, 328, 330, and 332 may next be fed to corresponding driver circuits326, 328, 330, and 332, which supply modulator driver signals that havea desired current and/or voltage for driving modulators present in PIC206, as discussed in greater detail below with reference to FIG. 5.

FIG. 5 illustrates transmitter or optical source OS-1 in greater detail.It is understood that remaining optical sources OS-1 to OS-10 have thesame or similar structure as optical source OS-1.

Optical source OS-1 may be provided on substrate 205 and may include alaser 508, such as a distributed feedback laser (DFB), that supplieslight to at least four (4) modulators 506, 512, 526 and 530. DFB 508 mayoutput continuous wave (CW) light at wavelength λ1 to a dual outputsplitter or coupler 510 (e.g. a 3 db coupler) having an input port andfirst and second output ports. Typically, the waveguides used to connectthe various components of optical source OS-1 may be polarizationdependent. A first output 510 a of coupler 510 supplies the CW light tofirst branching unit 511 and the second output 510 b supplies the CWlight to second branching unit 513. A first output 511 a of branchingunit 511 is coupled to modulator 506 and a second output 511 b iscoupled to modulator 512. Similarly, first output 513 a is coupled tomodulator 526 and second output 513 b is coupled to modulator 530.Modulators 506, 512, 526 and 530 may be, for example, Mach Zender (MZ)modulators. Each of the MZ modulators receives CW light from DFB 508 andsplits the light between two (2) arms or paths. An applied electricfield in one or both paths of a MZ modulator creates a change in therefractive index. In one example, if the relative phase between thesignals traveling through each path is 180° out of phase, destructiveinterference results and the signal is blocked. If the signals travelingthrough each path are in phase, the light may pass through the deviceand modulated with an associated data stream. The applied electric fieldmay also cause changes in the refractive index such that a phase oflight output from the MZ modulator is shifted or changed relative tolight input to the MZ modulator. Thus, appropriate changes in theelectric field can cause changes in phase of the light output from theMZ modulator.

Each of the MZ modulators 506, 512, 526 and 530 are driven with datasignals or drive signals supplied via driver circuits 326, 328, 330, and332, respectively. The CW light supplied to MZ modulator 506 via DFB 508and branching unit 511 is modulated in accordance with the drive signalsupplied by driver circuit 326. The modulated optical signal from MZmodulator 506 is supplied to first input 515 a of branching unit 515.Similarly, driver circuit 328 supplies further drive signals for drivingMZ modulator 512. The CW light supplied to MZ modulator 512 via DFB 508and branching unit 511 is modulated in accordance with the drive signalsupplied by driver circuit 328. The modulated optical signal from MZmodulator 512 is supplied to phase shifter 514 which shifts the phase ofthe signal 90° (π/2) to generate one of an in-phase (I) or quadrature(Q) components, which is supplied to second input 515 b of branchingunit 515. The modulated data signals from MZ modulator 506, whichincludes the other of the I and Q components, and from MZ modulator 512are supplied to polarization beam combiner (PBC) 538 via branching unit515.

Modulator driver 330 supplies a third drive signal for driving MZmodulator 526. MZ modulator 526, in turn, outputs modulated opticalsignals as one of the I and Q components. A polarization rotator 524 mayoptionally be disposed between coupler 510 and branching unit 513.Polarization rotator 524 may be a two port device that rotates thepolarization of light propagating through the device by a particularangle, usually an odd multiple of 90°. The CW light supplied from DFB108 is rotated by polarization rotator 124 and is supplied to MZmodulator 526 via first output 513 a of branching unit 513. MZ modulator526 then modulates the polarization rotated CW light supplied by DFB508, in accordance with drive signals from driver circuit 330. Themodulated optical signal from MZ modulator 526 is supplied to firstinput 517 a of branching unit 517.

A fourth drive signal is supplied by driver 332 for driving MZ modulator530. The CW light supplied from DFB 508 is also rotated by polarizationrotator 524 and is supplied to MZ modulator 530 via second output 513 bof branching unit 513. MZ modulator 530 then modulates the receivedoptical signal in accordance with the drive signal supplied by driver332. The modulated data signal from MZ modulator 530 is supplied tophase shifter 528 which shifts the phase the incoming signal 90° (π/2)and supplies the other of the I and Q components to second input 517 bof branching unit 517. Alternatively, polarization rotator 536 may bedisposed between branching unit 517 and PBC 538 and replaces rotator524. In that case, the polarization rotator 536 rotates both themodulated signals from MZ modulators 526 and 530 rather than the CWsignal from DFB 508 before modulation. The modulated data signal from MZmodulator 526 is supplied to first input port 538 a of polarization beamcombiner (PBC) 538. The modulated data signal from MZ modulator 530 issupplied to second input port 538 b of polarization beam combiner (PBC)538. PBC 538 combines the four modulated optical signals from branchingunits 515 and 517 and outputs a multiplexed optical signal havingwavelength λ1 to output port 538 c. In this manner, one DFB laser 108may provide a CW signal to four separate MZ modulators 506, 512, 526 and530 for modulating at least four separate optical channels by utilizingphase shifting and polarization rotation of the transmission signals.Alternatively, multiple CW light sources were used for each channelwhich increased device complexity, chip real estate, power requirementsand associated manufacturing costs.

Alternatively, splitter or coupler 510 may be omitted and DFB 508 may beconfigured as a dual output laser source to provide CW light to each ofthe MZ modulators 506, 512, 526 and 530 via branching units 511 and 513.In particular, coupler 510 may be replaced by DFB 508 configured as aback facet output device. Both outputs of DFB laser 508, from respectivesides 508-1 and 508-2 of DFB 508, are used, in this example, to realizea dual output signal source. A first output 508 a of DFB 508 supplies CWlight to branching unit 511 connected to MZ modulators 506 and 512. Theback facet or second output 508 b of DFB 508 supplies CW light branchingunit with 513 connected to MZ modulators 526 and 530 via path orwaveguide 543 (represented as a dashed line in FIG. 5). The dual outputconfiguration provides sufficient power to the respective MZ modulatorsat a power loss far less than that experienced through 3 dB coupler 510.The CW light supplied from second output 508 b is supplied to waveguide543 which is either coupled directly to branching unit 513 or topolarization rotator 524 disposed between DFB 508 and branching unit513. Polarization rotator 524 rotates the polarization of CW lightsupplied from second output 508 b of DFB 508 and supplies the rotatedlight to MZ modulator 526 via first output 513 a of branching unit 513and to MZ modulator 530 via second output 513 b of branching unit 513.Alternatively, as noted above, polarization rotator 524 may be replacedby polarization rotator 536 disposed between branching unit 517 and PBC538. In that case, polarization rotator 536 rotates both the modulatedsignals from MZ modulators 526 and 530 rather than the CW signal fromback facet output 508 b of DFB 508 before modulation.

The polarization multiplexed output from PBC 538, may be supplied tomultiplexer 208 in FIG. 2, along with the polarization multiplexedoutputs having wavelength λ2 to λ10 from remaining optical sources OS-2to OS-m. Multiplexer 208, which, as noted above, may include an AWG 204,supplies a group of optical signals to multiplexer 14 (see FIG. 1). Itis understood that PICs present in transmitter blocks 12-2 to 12-noperate in a similar fashion and include similar structure as PIC 206shown in FIGS. 2 and 5.

As noted above, optical signals output from transmitter block 12-1 arecombined with optical signals output from remaining transmitter blocks12-2 to 12-n onto optical communication path 16 and transmitted toreceive node 18 (see FIG. 1). In receive node 18, demultiplexer 20divides the incomings signal into optical signal groupings, such thateach grouping is fed to a corresponding one of receiver blocks 22-1 to22-n.

One of receiver blocks 22-1 is shown in greater detail in FIG. 6. It isunderstood that remaining receiver circuitry or blocks 22-2 to 22-n havethe same or similar structure as receiver block 22-1.

Receiver block 22-1 includes a receive PIC 602 provided on substrate604. PIC 602 includes an optical power splitter 603 that receivesoptical signals having wavelengths λ1 to λ10, for example, and suppliesa power split portion of each optical signal (each of which itself maybe considered an optical signal) to each of optical receivers OR-1 toOR-10. Each optical receiver OR-1 to OR-10, in turn, supplies acorresponding output to a respective one of circuit blocks CB3-1 toCB3-10 of ASIC 606, and each of circuit blocks CB3-1 to CB3-10, suppliesa respective output to a corresponding one of circuit blocks CB4-1 toCB4-10 of DSP 608. DSP 608, in turn, outputs a copy of data Data-1 inresponse to the input to circuit blocks CB4-1 to CB4-10.

Optical receiver OR-1 is shown in greater detail in FIG. 7. It isunderstood that remaining optical receivers OR-2 to OR-10 have the sameor similar structure as optical receiver OR-1. Optical receiver OR-1 mayinclude a polarization beam splitter (PBS) 702 operable to receivepolarization multiplexed optical signals λ1 to λ10 and to separate thesignal into X and Y orthogonal polarizations, i.e., vector components ofthe optical E-field of the incoming optical signals transmitted onoptical fiber medium 108. The orthogonal polarizations are then mixed in90 degree optical hybrid circuits (“hybrids”) 720 and 724 with lightfrom local oscillator (LO) laser 701 having wavelength λ1. Hybridcircuit 720 outputs four optical signals O1 a, O1 b, O2 a, O2 b andhybrid circuit 724 outputs four optical signals O3 a, O3 b, O4 a, and O4b, each representing the in-phase and quadrature components of theoptical E-field on X (TE) and Y (TM) polarizations, and each includinglight from local oscillator 701 and light from polarization beamsplitter 702. Optical signals O1 a, O1 b, O2 a, O2 b, O3 a, O3 b, O4 a,and O4 b are supplied to a respective one of photodetector circuits 709,711, 713, and 715. Each photodetector circuit includes a pair ofphotodiodes (such as photodiodes 709-1 and 709-2) configured as abalanced detector, for example, and each photodector circuit supplies acorresponding one of electrical signals E1, E2, E3, and E4.Alternatively, each photodetector may include one photodiode (such asphotodiode 709-1) or single-ended photodiode. Electrical signals E1 toE4 are indicative of data carried by optical signals λ1 to λ10 input toPBS 702. For example, these electrical signals may comprise fourbase-band analog electrical signals linearly proportional to thein-phase and quadrature components of the optical E-field on X and Ypolarizations.

FIG. 8 shows circuitry or circuit blocks CB3-1 and CB4-1 in greaterdetail. It is understood that remaining circuit blocks CB3-2 to CB3-10of ASIC 606 have a similar structure and operate in a similar manner ascircuit block CB3-1. In addition, it is understood that remainingcircuit blocks CB4-2 to CB4-10 of DSP 608 have a similar structure andoperation in a similar manner as circuit block CB4-1.

Circuit block CB3-1 includes known transimpedance amplifier andautomatic gain control (TIA/AGC 802) circuitry 802, 804, 806, and 808that receives a corresponding one of electrical signals E1, E2, E3, andE4. Circuitry 802, 804, 806, and 808, in turn, supplies correspondingelectrical signals or outputs to respective ones of anti-aliasingfilters 810, 812, 814, and 815, which, constitute low pass filters thatfurther block, suppress, or attenuate high frequency components due toknown “aliasing”. The electrical signals or outputs form filters 810,812, 814, and 816 are then supplied to corresponding ones ofanalog-to-digital converters (ADCs) 818, 820, 822, and 824.

ADCs 818, 820, 822, and 824, may sample at the same or substantially thesame sampling rate as DACs 310, 312, 314, and 316 discussed above.Preferably, however, circuit block CB4-1 and DSP 608 have an associatedsampling rate that is less than the DAC sampling rate. At such a highsampling rate, DSP 608 and its associated circuitry or circuits, wouldconsume excessive power and would require a relatively complex design.Accordingly, in order to reduce the rate that samples are supplied toand processed by DSP 608, FIFO interpolation and filter circuits 826 and828 are provided to provide samples at a lower sampling rate than thatassociated with DACs 818, 820, 822, and 824. The operation and structureof FIFO interpolation and filter circuits are described in greaterdetail in U.S. patent application Ser. No. 12/791,694 titled “Method,System, And Apparatus For Interpolating An Output Of AnAnalog-To-Digital Converter”, filed Jun. 1, 2010, the entire contents ofwhich are incorporated herein by reference.

The electrical signals or outputs of circuits 826 and 828 are providedto filters, such as digital filters 830 and 832, which may performspectral shaping in a known manner similar to that discussed above inconnection with filters 302 and 304 (see FIG. 3) to select a datastream, information stream, or data within a bandwidth associated withfilters 830 and 832. Such data or information stream also corresponds toone of the optical signals, e.g., the optical signal having wavelengthλ1, polarization, and I or Q component. In addition, the outputs offilter circuits 830 and 832 are next fed to processor circuitry 834 thatperforms equalization, carrier recovery, and other known demodulationtasks. CB4-1, as noted above, then outputs a copy of data Data-1 fromreceive node 18.

As noted above, electronic or digital filters in receive node 18, suchas filters 830 and 832 electronically separate the data carried bywavelength λ1, as opposed to optically demultiplexing such data. Inorder to minimize errors in such data attributable to aliasing intransmit node 11, up-sampling of the data output from filters 302 and304 is performed so that the DACs in transmit node 11 operate at ahigher sampling rate than the sampling rate associated with filter 302and 304. As a result, high frequency harmonics are spectrally spacedfrom the bandwidth associated with the data to be transmitted, and suchharmonics can then be readily filtered by roofing filters 318 and 320,for example. Thus, upsampling in transmit node 11 may facilitatedetection and electrical filtering of data carried by an optical signalwithout optical demultiplexing of individual optical channels.

Thus, for example, as shown in FIG. 9 a, without upsampling, thefrequency spectrum associated with data output from spectral shapingfilter 302, for example, may be represented by curve 904, and the highfrequency harmonics are represented by spectrum 903. The filtercharacteristic associated with roofing filter 318 is represented bycurve 902, which, as shown in FIG. 9 a extends over portion 907 of thehigh frequency harmonics spectrum 903. Thus, portions of the highfrequency harmonics are detected as noise in the electrical signalsgenerated by photodiodes in receive node 18, such as those inphotodetector circuit 709 (see FIG. 7).

Consistent with the present disclosure, however, when upsampling iscarried out, e.g., by FIFO and interpolation filter 306, the higherfrequency harmonic spectrum 903 is spaced farther from data bandwidth904 by a range of frequencies labeled 915 in FIG. 9 b. Here, thespectrum or characteristic 902 of roofing filter 318 does not overlap orextend over any portion or any substantial portion of high frequencyharmonic spectrum 903. As a result, such harmonics are suppressed in theelectrical signals output from photodetector circuit 709, such thaterrors are reduced and adequate detection of electrically filtered datacan be obtained. In addition, roofing filter 318 blocks frequenciesassociated with adjacent channels, such that channels may be spectrallyspaced closer to one another.

Since, as noted above, data associated with an individual wavelength maybe electronically filtered or selected, as opposed to being opticallydemultiplexed, the bandwidth associated with such data, and, thus thechannel bandwidth may be varied to accommodate different data rates andmodulation formats. In addition, depending on fiber type, channelwavelength and other system parameters, a particular modulation formatmay provide an optimized (i.e., maximum) capacity for a given distance.Accordingly, a laser in transmit node 11 may be tuned to that channelwavelength and an electronic filter in receive node 18 may be adjustedto accommodate the bandwidth associated with the desired modulationformat. Moreover, depending on system parameters, an optimized number ofchannels having different modulation formats may be provided in order toprovide maximum capacity for a given distance.

As an example, assuming an input data rate,fbaud=28 GHz, and a samplingrate of DSP 202 of 37.33 GHz (DSP sampling rate=P×fbaud, where P=4/3).Here, the rolloff can be from 0.1 to 0.33, and programmable oradjustable, as noted above. As further discussed above, the outputs ofDSP 202 are interpolated from P*fbaud to Q*fbaud (Q*fbaud being thesampling rate of DAC 310), for example, where 1<P<Q≦2. Further, in thisexample, analog roofing filter 318, for example, may suppress thedigitally sampled spectrum beyond the desired baseband signal BW of18.67 GHz (see harmonic spectrum 903 discussed above). As a result, thesampling rate of DAC 310 may be set to be higher than that of DSP 202.For example, when Q=2, it is T/2-spaced sampling (T being the period orduration of a symbol or 1/fbaud). The higher the sampling rate of DAC310, the more separation of the harmonic spectra 903, and as such it iseasier to filter out the harmonic spectra. Furthermore, programmable oradjustable rolloff is more readily achievable.

In the above example, a rolloff of less than 0.33 and greater than 0.1can be obtained. In addition, roofing filter 318, for example, can thenbe designed to tailor to the highest α and the sampling rate of DAC 310.It is noted that more heat may be generated by ASIC 204 at higher DACsampling rates. Accordingly, less than T2-spaced sampling may be choses,e.g., 5/3*fbaud, which is 46.67 GHz.

As noted above, upsampling shifts the harmonic spectrum 903 to a highersampling frequency (46.67 GHz as an example), thereby creating aspectrum gap between the desired signal spectrum and the harmonicspectrum. This not only makes the design of roofing filter 318 mucheasier, but also facilitates or realizes a programmable or adjustablerolloff. In addition, as discussed in greater detail below withreference to FIG. 8, similar interpolation may be done in the receivenode, whereby data samples output from an analog-to-digital converter(ADC) having a sampling rate of at Q*fbaud may be down-sampled to alower sampling rate (P*fbaud) for further signal processing by a DSP.Here, coefficients P and Q may be the same in both the receive andtransmit nodes, thereby easing design of an analog anti-aliasing filterfor programmable rolloff in the receive node (see discussion below).

Examples of systems having a variable electrical or digital filterbandwidth will next be described with reference to FIGS. 10, 11 a-11,12, 13, 14, 15 a-15 b, and 16.

FIG. 10 illustrates an example of an optical communication system 1000consistent with an additional aspect of the present disclosure. System1000 includes forward error correction (FEC) encoder circuits 1004-1 to1004-n that FEC encode data (such as Data-1 to Data-n) supplied totransmit blocks 12-1 to 12-n. After propagating through system 1000, thedata carried by optical signals output from transmit blocks 12-1 to 12-nis demultiplexed according to optical channel groupings in receive node18, processed (as noted above), and then output to FEC decoder circuits1008-1 to 1008-n. FEC decoder circuits 1008-1 to 1008-n, in turn, decodethe outputs from RX blocks 22-1 to 22-n to supply copies of Data-1 toData-n.

Consistent with the example shown in FIG. 10, a first channelpropagating along optical communication 16 may be significantly degradedand may required more FEC encoding than a second channel propagatingalong path 16. As a result, frames 1120 of data carried by the firstchannel may resemble that shown in FIG. 11 a, which include a payloadportion 1122 and overhead portion 1224 with additional error correctingor FEC bytes (FIG. 1120). Accordingly, the amount of data (payload andoverhead) carried by frames 1120 is increased, and thus the data, symbolor baud rate associated with frames 1120 is also increased. As a result,frames 1120 have an associated increased channel bandwidth, as indicatedby curve 1109, as well as a correspondingly increased filter bandwidth,as indicated by curve 1110 and cutoff frequency fa (FIG. 11 a). On theother hand, a second channel that is not significantly degraded may havea frame (see frame 1140, FIG. 11 d) with fewer FEC bytes in overhead1144. The amount of data carried by such a channel (data, symbol or baudrate) is thus less than that of the first channel, and, therefore, thechannel bandwidth 1111, as well as the filter bandwidth 1112 is alsoless (FIG. 11 c).

Therefore, consistent with an aspect of the present disclosure, thosechannels requiring less FEC encoding have a narrower bandwidth (e.g.,bandwidth 1112 and cutoff frequency fc which is less than fa) and may bespectrally spaced closer to one another than those channel requiringmore FEC encoding (e.g., bandwidth 1110) and more bandwidth. Theelectrical filtering discussed above (e.g., filtering in the electrical,as opposed to the optical domain) can readily accommodate such varyingbandwidths, such that, for example, if channel degradation improves,fewer FEC bytes are provided in the frame overhead, and the resultingbandwidth of the channel can be rendered more narrow to accommodateadditional channels.

In one example, different control inputs CI-1 may be supplied to FECencoder circuit 1004-1 to adjust the amount of FEC encoding or thenumber of error correcting bits or bytes output therefrom and includedin each frame output from Tx block 12-1. In addition, different controlinput CI-n may be supplied to control or adjust the number of errorcorrecting bits or bytes output therefrom and included in each frameoutput from Tx block 12-n. In another example, the number of errorcorrecting bits in each frame output from Tx block 1004-1 (as well asoutput FEC encoder circuit 1004-1) may be less than the number of errorcorrecting bits in each frame output from Tx block 1004-2 (as well asoutput from FEC encoder circuit 1004-n). It is understood that thecontrol input may be any appropriate signal, information, or data thatis supplied to the encoder circuit, such that the number of errorcorrecting bits in each frame is changed in response to such signal,information or data.

FIG. 12 illustrates another example in switch demultiplexer 20 shown inFIG. 1 is replaced by a known wavelength selective switch (WSS). Asgenerally understood and depending on system requirements, WSS 1210 maybe configured to output first optical signals from path 16 havingwavelengths within a first range or optical bandwidth from a first port1210-1 and output wavelengths within a second range from a second port1210-n. Consistent the present disclosure, additional optical signalshaving associated bandwidths may be supplied from either one or bothports 1210-1 and 1210-n in order to maximize the number of opticalsignals supplied from each port. Alternatively, optical signalsmodulated in accordance with a modulation format requiring additionalbandwidth or optical signals requiring additional FEC overhead may beoutput through a port of WSS 1210 that has a relatively large opticalbandwidth, while optical signals having a relative narrow bandwidth maybe output through a port of WSS 1210 that has a narrow opticalbandwidth.

In the above examples, the optical signals output from transmit blocks12-1 to 12-n are “banded” in that the wavelength of each optical channelsupplied from one transmit block is not between the wavelengths ofoptical signals output from another transmit block. Consistent withanother aspect of the present disclosure, however, multiplexer 14 may bereplaced by a known optical interleaver 1302 in system 1300 tospectrally interleave the optical channels that propagate along opticalpath 16. In addition, demultiplexer 20 may be replaced by a knownde-interleaver 1304. Accordingly, in this example, the wavelengths ofoptical signals output from transmit block 12-1 are relatively far apartfrom each other, spectrally, to accommodate the wavelengths of opticalsignals from other transmit blocks 12-1.

FIG. 14 illustrates another example, in which selected transmit blocks,such as transmit block 1402-1, output optical signals, which have beenmodulated in accordance with a first modulation format, and othertransmit blocks, such as transmit block 1402-n, that output modulatedoptical signals having a second modulation format. As generallyunderstood, different modulation format may have wider or narrowerbandwidths than other modulation modulation formats. For example, thefirst modulation format may be a binary phase shift keyed (BPSK)modulation format and the second modulation format may be a quadraturephase shift keyed (QPSK) modulation format.

In one example, in response to a first control signal supplied to TXblock 1402-1, TX block 1402-1 may output a first optical signalmodulated in accordance with a first modulation format, such as a BPSKmodulation format. Moreover, in response to a second control signalsupplied to TX block 1402-1, TX block 1402-1 may output a second opticalsignal modulated in accordance with a second modulation format, such asQPSK. Further, in response to a third control signal supplied to TXblock 1402-n, TX block 1402-n may output a third optical signalmodulated in accordance with the first modulation format, such as theBPSK modulation format. Moreover, in response to a fourth control signalsupplied to TX block 1402-n, TX block 1402-n may output a second opticalsignal modulated in accordance with a second modulation format, such asQPSK. Alternatively, in a similar manner, control signals may besupplied such that the TX blocks selectively output 8-QAM or highermodulation format optical signals.

As further shown in FIG. 14, a grouping of optical signals having thefirst modulation format may be input to and supplied by demultiplexer 20to receive block 1404-1 and optical signals having the second modulationformat may be input to and supplied by demultiplexer 20 to receive block1404-1. Electronic or digital filters in the transmit (1402) and receive(1404) may have adjustable bandwidths to select data carried by opticalsignal modulated in accordance with the first or second bandwidths, asthe case may be.

For example, as shown in FIG. 15 a, electronic or digital filterbandwidth 1504 (having cutoff frequency fa') may be relatively large toselect and/or filter the channel bandwidth or modulation spectrum 1502of a BPSK modulated optical signal. On the other hand, electronic ordigital filter bandwidth 1508 (having cutoff frequency fc′) may beadjusted to be relatively narrow to accommodate or filter relativelynarrow bandwidth or modulation spectrum 1506 associated with a QPSKoptical signal carrying data at the same rate, for example, as the BPSKmodulated optical signal. It is understood, however, that if the BPSKmodulated optical signal carries data at half of the data rateassociated with bandwidth spectrum 1502 (a so-called “half-rate”signal), spectrum 1502 would have the same or substantially the samebandwidth as spectrum 1506.

Consistent with a further aspect of the present disclosure, transmitblock 1402-1 may output 8-QAM modulated optical signals having arelatively narrow bandwidth. For example, as shown in FIG. 15 c,electronic or digital filter bandwidth 1510 (having cutoff frequencyfc″) may be adjusted to be relatively narrow to select and/or filter thechannel bandwidth or modulation spectrum 1508 of an 8-QAM modulatedoptical signal. For a given data rate, the 8-QAM modulated opticalsignal will have an associated bandwidth that is less than a QPSKmodulated optical signal carrying data at the same rate as the 8-QAMmodulated optical signal. Typically, for a given data rate, thebandwidth of n-QAM modulated optical signals (where n=16, 32, . . . )decreases with increasing n.

In a further example, FIG. 16 illustrates a channel plan 1600 includingwavelengths, which are represented by solid arrows 1602, 1604, 1606,1608, 1610, 1612, 1614, 1616, and 1618. Channel plan 1600 may representthe wavelengths of optical signals supplied by the Tx blocks discussedabove and received by the Rx blocks, also discussed above. Here, thespectral spacing between adjacent ones of these wavelengths is less than50 GHz, and preferably less than 25 GHz. Moreover, the spacing betweenpairs of adjacent wavelengths may differ. For example, the spacingbetween wavelengths represented by arrows 1602 and 1604 is differentthan the spacing between wavelengths represented by arrows 1608 and1610.

As further shown in FIG. 16, an International Telecommunications Union(ITU) grid having both 25 and 50 Ghz spacing (as represented by dashedarrows 1601, 1603, 1605, 1607, 1609, and 1611) is superimposed onchannel plan 1600 to illustrate that the spacing between the opticalchannels represented by the solid arrows is different, and non-uniform,relative the spacing associated with the ITU grid. Such non-uniformchannel spacing may result from different electrical filter bandwidths.

Thus, for example, if certain channels are more susceptible totransmission impairments over longer distances than other channels, suchchannels may be modulated in accordance with a modulation format, suchas BPSK that is less susceptible to such impairments. Other channelsthat do not suffer from the transmission impairments over such longdistances may be transmitted in accordance with a more spectrallyefficient modulation format, such as QPSK or 8-QAM. Alternatively, suchoptical channels may be transmitted over shorter distances. Since thebandwidths associated with such modulation formats may differ, forexample, BPSK modulated optical signals have a higher bandwidth thanQPSK and 8-QAM modulated optical signals carrying data at the same rate,electronic filters having bandwidths tailored for each modulation formatmay be readily provided. The spectral spacing between channels may thusbe non-uniform whereby BPSK modulated optical signals, for example, maybe spectrally farther apart than QPSK or 8-QAM modulated opticalsignals. Moreover, channels may be provided spectrally closer to oneanother than would otherwise be the case if the channels were to conformto an ITU grid.

In another example, one or more of the optical signals discussed abovemay be modulated to carry data at a data rate greater than or equal to40 Gbits/second. Moreover, consistent with a further aspect of thepresent disclosure, one or more of the optical signal may be modulatedto carry data at a data rate greater than or equal to 100 Gbits/second,and such 100 Gbits/second optical signals may be transmitted along with40 Gbit/second optical signals.

A method consistent with a further aspect of the present disclosure willnext be described with reference to flowchart 1700 shown in FIG. 17. Ina first step 1702, a data carrying capacity of an optical link isidentified. The optical link, such as optical link or system 100, mayinclude transmit node 11 and receive node 18, as noted above inconnection with the description of FIG. 1. As further noted above, suchan optical link is configured to carry a plurality of optical signals,each of which having a corresponding one of a plurality of wavelengths.The method further includes, for each of the plurality of opticalsignals, identifying one of a plurality of modulation formats, such asBPSK, QPSK, and 8-QAM, and one of a plurality of data rates, such as 40Gbits/second and 100 Gbits/second (step 1704). In addition, the methodincludes determining a spectral spacing between adjacent ones of theplurality of wavelengths (step 1706). As a result of the above steps,when each of the plurality of optical signals is modulated in accordancewith a corresponding identified one of the plurality of modulationformats, carries data at a corresponding one of the plurality of datarates, and the plurality of wavelengths conform to the spectral spacing,the data carrying capacity of the optical link is optimized and thedesired capacity is obtained. As further noted above, the channelspacing in such an optimized system or link may be less than 25 GHz andmay not conform to an ITU grid.

As further discussed above, optical communication systems may beprovided with electrical filters having varying bandwidths. As a result,optical signals having different modulation formats, different levels ofFEC encoding, and close, non-uniform channel spacings may transmittedover different distances and detected so that system capacity may beoptimized, and thus increased.

Other embodiments will be apparent to those skilled in the art fromconsideration of the specification. It is intended that thespecification and examples be considered as exemplary only, with a truescope and spirit of the invention being indicated by the followingclaims.

1. An apparatus, comprising: a plurality of optical transmitters, eachof which being configured to supply a corresponding one of a pluralityof first optical signals, each of the plurality of first optical signalshaving a corresponding one of a plurality of wavelengths and carrying acorresponding one of a plurality of data streams; an optical combinerconfigured to combine the plurality of first optical signals onto anoptical path; a photodiode, the photodiode configured to receive aportion of each of the plurality of first optical signals and supply afirst electrical signal; a local oscillator laser, at least a portion ofa second optical signal output from the local oscillator is supplied tothe photodiode; and circuitry configured to receive the first electricalsignal and supply a second electrical signal in response to the firstelectrical signal, the circuitry including an electronic filter having avariable bandwidth, the second electrical signal carrying one of theplurality of data streams.
 2. An apparatus in accordance with claim 1,wherein each of the plurality of wavelengths is spectrally spaced fromone another by a spacing less than 50 GHz.
 3. An apparatus in accordancewith claim 2, wherein the spacing is less than 25 GHz.
 4. An apparatusin accordance with claim 1, wherein first and second adjacent ones ofthe plurality of wavelengths are separated from each other by a firstspacing and second and third adjacent ones of the plurality ofwavelengths are separated from each other by a second spacing differentthan the first spacing.
 5. An apparatus in accordance with claim 1,wherein said one of the plurality of data streams has an associated datarate, the data rate being greater than or equal to 40 Gbits/second. 6.An apparatus in accordance with claim 5, wherein the data rate isgreater than or equal to 100 Gbits/second.
 7. An apparatus in accordancewith claim 1, wherein each of the plurality of wavelengths is spectrallyspaced from one another by a spacing, the spacing different than anInternational Telecommunications Union (ITU) channel spacing.
 8. Asystem, comprising: a first transmitter including: a first digitalsignal processor circuit configured to receive input data, the digitalsignal processor being configured to sample the input data at a firstsampling rate, and spectrally shape the input data to supply spectrallyshaped data as a first plurality of data samples at the first samplingrate, a first interpolation circuit that receives the first plurality ofdata samples at the first sampling rate and outputs the spectrallyshaped data as a second plurality of data samples at a second samplingrate greater than the first sampling rate, a digital-to-analog convertercircuit configured to receive the second plurality of data samples andgenerate an analog signal, a modulator circuit, and a laser configuredto supply light to the modulator circuit, the modulator circuit beingconfigured to modulate the light to supply a first modulated opticalsignal in response to the analog signal, a second transmitter supplyinga second modulated optical signal; a combiner configured to combine thefirst modulated optical signal with the second modulated optical signalonto an optical communication path; and a receiver coupled to theoptical communication path, the receiver including: a photodiode thatreceives portions the first and second modulated optical signals, thephotodiode supplying an electrical output, a low-pass filter thatsupplies a filtered output in response to the electrical output, ananalog-to-digital converter configured to sample the filtered output atthe second sampling rate to generate a plurality of third data samples,a second interpolation circuit configured to receive the plurality ofthird data samples and supply a plurality of fourth data samples at athird sampling rate less than the second sampling rate, and a seconddigital signal processor circuit configured to receive the plurality offourth data samples.
 9. A system in accordance with claim 8, wherein thefirst sampling rate is the same as the third sampling rate.
 10. A systemin accordance with claim 8, wherein the first and second modulatedoptical signals having first and second wavelengths, respectively.
 11. Asystem in accordance with claim 8, wherein the system includes aplurality of transmitters, the first transmitter and the secondtransmitter being respective first and second ones of the plurality ofoptical transmitters, each of the plurality of transmitters supplying acorresponding one of a plurality of optical signals, each of whichhaving a corresponding one of a plurality of wavelengths, the firstoptical signal and the second optical signal being respective first andsecond ones of the plurality of optical signals, and the first andsecond wavelengths being first and second ones of the plurality ofwavelengths, each of the plurality of wavelengths is spectrally spacedfrom one another by a spacing less than 50 GHz.
 12. A system inaccordance with claim 11, wherein the spacing is less than 25 GHz.
 13. Asystem in accordance with claim 8, wherein the system includes aplurality of transmitters, the first transmitter and the secondtransmitter being respective first and second ones of the plurality oftransmitters, each of the plurality of transmitters supplying acorresponding one of a plurality of optical signals, each of whichhaving a corresponding one of a plurality of wavelengths, the firstoptical signal and the second optical signals being respective first andsecond ones of the plurality of optical signals, and the first andsecond wavelengths being first and second ones of the plurality ofwavelengths, wherein each of a first adjacent pair of the plurality ofwavelengths is separated from each other by a first spacing and each ofa second adjacent pair of the plurality of wavelengths is separated fromeach other by a second spacing different than the first spacing.
 14. Asystem in accordance with claim 8, wherein the input data has anassociated data rate, the data rate being greater than or equal to 40Gbits/second.
 15. An apparatus in accordance with claim 14, wherein thedata rate is greater than or equal to 100 Gbits/second.
 16. An apparatusin accordance with claim 8, wherein the apparatus includes a pluralityof optical transmitters, the first optical transmitter and the secondoptical transmitter being respective first and second ones of theplurality of optical transmitters, each of the plurality of opticaltransmitters supplying a corresponding one of a plurality of opticalsignals, each of which having a corresponding one of a plurality ofwavelengths, the first optical signal and the second optical signalbeing respective first and second ones of the plurality of opticalsignals, and the first and second wavelengths being first and secondones of the plurality of wavelengths, wherein each of the plurality ofwavelengths is spectrally spaced from one another by a spacing, thespacing being different than an International Telecommunications Union(ITU) channel spacing.
 17. A method, comprising: identifying a datacarrying capacity of an optical link including a transmit node and areceive node, the optical link being configured to carry a plurality ofoptical signals, each of which having a corresponding one of a pluralityof wavelengths; for each of the plurality of optical signals,identifying one of a plurality of modulation formats and one of aplurality of data rates; determining a spectral spacing between adjacentones of the plurality of wavelengths, such that when each of theplurality of optical signals is modulated in accordance with acorresponding identified one of the plurality of modulation formats,carries data at a corresponding one of the plurality of data rates, andthe plurality of wavelengths conform to the spectral spacing, the datacarrying capacity of the optical link is obtained.
 18. A method inaccordance with claim 17, wherein the plurality of modulation formatsincludes at least one of BPSK, QPSK, and 8-QAM.
 19. A method inaccordance with claim 17, wherein the plurality of optical wavelengthsdo not conform to an ITU grid.
 20. A method in accordance with claim 17,wherein the spectral spacing is less than 25 GHz.